Network Working Group D. Katz
Request for Comments: 3373 Juniper Networks, Inc.
Category: Informational R. Saluja
Tenet Technologies
September 2002
Three-Way Handshake forIntermediate System to Intermediate System (IS-IS)Point-to-Point Adjacencies
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The IS-IS routing protocol (ISO 10589) requires reliable protocols at
the link layer for point-to-point links. As a result, it does not
use a three-way handshake when establishing adjacencies on point-to-
point media. This paper defines a backward-compatible extension to
the protocol that provides for a three-way handshake. It is fully
interoperable with systems that do not support the extension.
Additionally, the extension allows the robust operation of more than
256 point-to-point links on a single router.
This extension has been implemented by multiple router vendors; this
paper is provided as information to the Internet community in order
to allow interoperable implementations to be built by other vendors.
1. Terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119.
2. Introduction
The IS-IS protocol [1] assumes certain requirements stated in ISO
10589 (section 6.7.2) for the operation of IS-IS over point-to-point
links and hence provides only a two-way handshake when establishing
Katz & Saluja Informational [Page 1]

RFC 3373 Three-Way Handshake for IS-IS September 2002
adjacencies on point-to-point links. The protocol does not operate
correctly if these subnetwork requirements for point-to-point links
are not met. The basic mechanism defined in the standard is that
each side declares the other side to be reachable if a Hello packet
is heard from it. Once this occurs, each side then sends a Complete
Sequence Number PDU (CSNP) to trigger database synchronization.
Three failure modes are known. First, if the link goes down and then
comes back up, or one of the systems restarts, and the CSNP packet is
lost, and the network has a cut set of one through the link, the link
state databases on either side of the link will not synchronize for a
full LSP refresh period (up to eighteen hours).
A second, more serious failure, is that if the link fails in only one
direction, the failure will only be detected by one of the systems.
Normally only one of the two systems will announce the adjacency in
its link state packets, and the SPF algorithm will thus ignore the
link. However, if there are two parallel links between systems and
one of them fails in one direction, SPF will still calculate paths
between the two systems, and the system that does not notice the
failure will attempt to pass traffic down the failed link (in the
direction that does not work).
The third issue is that on some physical layers, the
interconnectivity between endpoints can change without causing a
link-layer-down condition. In this case, a system may receive
packets that are actually destined for a different system (or a
different link on the same system). The receiving system may end up
thinking that it has an adjacency with the remote system when in fact
the remote system is adjacent with a third system.
The solution proposed here ensures correct operation of the protocol
over unreliable point-to-point links. As part of the solution to the
three-way handshaking issue, a method is defined to remove the
limitation of 255 point-to-point interfaces imposed by IS-IS [1].
This method is more robust than the ad hoc methods currently in use.
3. Overview of Extensions3.1 Handshaking
The intent is to provide a three-way handshake for point-to-point
adjacency establishment in a backward compatible fashion. This is
done by providing an optional mechanism that allows each system to
report its adjacency three-way state; this allows a system to only
declare an adjacency to be up if it knows that the other system is
receiving its IS-IS Hello (IIH) packets.
Katz & Saluja Informational [Page 2]

RFC 3373 Three-Way Handshake for IS-IS September 2002
The adjacency three-way state can be one of the following types:
Down
This is the initial point-to-point adjacency three-way state. The
system has not received any IIH packet containing the three-way
handshake option on this point-to-point circuit.
Initializing
The system has received IIH packet containing the three-way
handshake option from a neighbor but does not know whether the
neighbor is receiving its IIH packet.
Up
The system knows that the neighbor is receiving its IIH packets.
The adjacency three-way state that is reported by this mechanism is
not equal or equivalent to the adjacency state that is described in
ISO 10589 [1]. If this mechanism is supported then an adjacency may
have two states, its state as defined in ISO 10589 [1], and its
three-way state. For example according to ISO 10589 [1] receipt of
an ISH will cause an adjacency to go to Initializing state; however
receipt of an ISH will have no effect on the three-way state of an
adjacency, which remains firmly Down until it receives an IIH from a
neighbor that contains the three-way handshaking option.
In addition, the neighbor's system ID and (newly-defined) extended
circuit ID are reported in order to detect the case where the same
stream is being received by multiple systems (only one of which can
talk back).
The mechanism is quite similar to the one defined in the Netware Link
Services Protocol (NLSP) [2], a variant of IS-IS used for routing IPX
traffic. The difference between this mechanism and the one used in
NLSP is the location where the information is carried (NLSP uses two
of the reserved bits in the IIH header, whereas this solution adds a
separate option to the IIH), and the presence of the neighbor's
system ID and circuit ID. In theory, using the reserved header bits
should be backward compatible, since systems are supposed to ignore
them. However, it was felt that this was risky, as the use of
untested mechanisms such as this have led to problems in the past in
other protocols. New option codes, on the other hand, have been
demonstrated to work properly, as the deployment of Integrated IS-IS
for IP [3] has done exactly this.
The new mechanism only comes into play when the remote system
includes the new option in its IIH packet; if the option is not
present, it is assumed that the system does not support the new
mechanism, and so the old procedures are used.
Katz & Saluja Informational [Page 3]

RFC 3373 Three-Way Handshake for IS-IS September 20023.2 More Than 256 Interfaces
The IS-IS specification has an implicit limit of 256 interfaces, as
constrained by the eight bit Circuit ID field carried in various
packets. Moderately clever implementors have realized that the only
true constraint is that of 256 LAN interfaces, and for that matter
only 256 LAN interfaces for which a system is the Designated IS.
This is because the only place that the circuit ID is advertised in
LSPs is in the pseudonode LSP ID.
Implementors have treated the point-to-point Circuit ID number space
as being independent from that of the LAN interfaces, since these
Circuit IDs appear only in IIH PDUs and are only used for detection
of a change in identity at the other end of a link. More than 256
point-to-point interfaces have been supported by sending the same
circuit ID on multiple interfaces. This reduces the robustness of
the ID change detection algorithm, since it would then be possible to
switch links between interfaces on a system without detecting the
change.
Since the Circuit ID is an integral part of the new handshaking
mechanism, a backward compatible mechanism for expanding the circuit
ID number space is included in this specification.
4. Details4.1 Syntax
A new IS-IS Option type, "Point-to-Point Three-Way Adjacency", is
defined:
x Type - 0xF0 (decimal 240)
x Length - total length of the value field (1 to 17 octets)
x Value -
No. of Octets
+-----------------------------------+
| Adjacency Three-Way State | 1
+-----------------------------------+
| Extended Local Circuit ID | 4
+-----------------------------------+
| Neighbor System ID | ID Length
+-----------------------------------+
| Neighbor Extended Local Circuit ID| 4
+-----------------------------------+
Katz & Saluja Informational [Page 4]

RFC 3373 Three-Way Handshake for IS-IS September 2002
Adjacency Three-Way State
The adjacency three-way state of the point-to-point adjacency. The
following values are defined:
0 - Up
1 - Initializing
2 - Down
Extended Local Circuit ID
Unique ID assigned to this circuit when it is created by this
Intermediate system.
Neighbor System ID
System ID of neighbor Intermediate system if known. The length of
this field is equal to "ID Length" of IIH PDU described in section
"Point-to-point IS to IS hello PDU" (section 9.7 of [1]).
Neighbor Extended Local Circuit ID
Extended Local Circuit ID of the other end of the point-to-point
adjacency if known.
Any system that supports this mechanism SHALL include this option in
its Point-to-Point IIH packets.
Any system that does not understand this option SHALL ignore it, and
(of course) SHALL NOT include it in its own IIH packets.
Any system that supports this mechanism MUST include Adjacency
Three-Way State field in this option. The other fields in this
option SHOULD be included as explained below in section 3.2.
Any system that is able to process this option SHALL follow the
procedures below.
4.2 Elements of Procedure
The new handshake procedure is added to the IS-IS point-to-point IIH
state machine after the PDU acceptance tests have been performed.
Although the extended circuit ID is only used in the context of the
three-way handshake, it is worth noting that it effectively protects
against the unlikely event where a link is moved to another interface
on a system that has the same local circuit ID, as the received PDUs
will be ignored (via the checks defined below) and the existing
adjacency will fail.
Katz & Saluja Informational [Page 5]

RFC 3373 Three-Way Handshake for IS-IS September 2002
Add a clause e) to the end of section "Receiving ISH PDUs by an
intermediate system" (section 8.2.2 of [1]):
Set the state to be reported in the Adjacency Three-Way State
field of the Point-to-Point Three-Way Adjacency option to Down.
Add a clause e) to the end of section "Sending point-to-point IIH
PDUs" (section 8.2.3 of [1]):
The IS SHALL include the Point-to-Point Three-Way Adjacency option
in the transmitted Point-to-Point IIH PDU. The current three-way
state of the adjacency with its neighbor on the link (as defined
in new section 8.2.4.1.1 introduced later in the document) SHALL
be reported in the Adjacency Three-Way State field. If no
adjacency exists, the state SHALL be reported as Down.
The Extended Local Circuit ID field SHALL contain a value assigned
by this IS when the circuit is created. This value SHALL be
unique among all the circuits of this Intermediate System. The
value is not necessarily related to that carried in the Local
Circuit ID field of the IIH PDU.
If the system ID and Extended Local Circuit ID of the neighboring
system are known (in adjacency three-way state Initializing or
Up), the neighbor's system ID SHALL be reported in the Neighbor
System ID field, and the neighbor's Extended Local Circuit ID
SHALL be reported in the Neighbor Extended Local Circuit ID field.
Add a section 8.2.4.1.1, "Three-Way Handshake", immediately prior to
section "IIH PDU Processing" (section 8.2.4.2 of [1]):
A received Point-to-Point IIH PDU may or may not contain the
Point-to-Point Three-Way Adjacency option. If it does not, the
link is assumed to be functional in both directions, and the
procedures described in section 8.2.4.2 are followed.
If the option is present and contains invalid Adjacency Three-Way
State, the PDU SHALL be discarded and no further action is taken.
If the option with a valid Adjacency Three-Way State is present,
the Neighbor System ID and Neighbor Extended Local Circuit ID
fields, if present, SHALL be examined. If they are present, and
the Neighbor System ID contained therein does not match the local
system's ID, or the Neighbor Extended Local Circuit ID does not
match the local system's extended circuit ID, the PDU SHALL be
discarded and no further action is taken.
Katz & Saluja Informational [Page 6]

RFC 3373 Three-Way Handshake for IS-IS September 2002
If the Neighbor System ID and Neighbor Extended Local Circuit ID
fields match those of the local system, or are not present, the
procedures described in section 8.2.4.2 are followed with
following changes:
a) In section 8.2.4.2 a and b, the action "Up" from state tables
5, 6, 7 and 8 may create a new adjacency but the three-way
state of the adjacency SHALL be Down.
b) If the action taken from section 8.2.4.2 a or b is "Up" or
"Accept", the IS SHALL perform the action indicated by the
new adjacency three-way state table below, based on the
current adjacency three-way state and the received Adjacency
Three-Way State value from the option. (Note that the
procedure works properly if neither field is ever included.
This provides backward compatibility to an earlier version of
this option.)
Received Adjacency Three-Way State
Down Initializing Up
-------------------------------------------------
Down | Initialize Up Down
|
adj Initializing | Initialize Up Up
three |
-way Up | Initialize Accept Accept
state |
|
Adjacency Three-Way State Table
If the new action is "Down", an adjacencyStateChange(Down)
event is generated with the reason "Neighbor restarted" and the
adjacency SHALL be deleted.
If the new action is "Initialize", no event is generated and
the adjacency three-way state SHALL be set to "Initializing".
If the new action is "Up", an adjacencyStateChange(Up)
event is generated.
c) Skip section 8.2.4.2 c and d.
d) If the new action is "Initialize", "Up" or "Accept", follow
section 8.2.4.2 e.
Katz & Saluja Informational [Page 7]

RFC 3373 Three-Way Handshake for IS-IS September 20029. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Katz & Saluja Informational [Page 9]